1. Field of the Invention
The present invention relates to membrane separation at relatively high recovery of a gas that is present in a gas mixture at a relatively low concentration. More particularly, the present invention relates to membrane separation at relatively high recovery of Helium that is present in natural gas at a relatively low concentration.
2. Related Art
For relatively rare and/or costly gases, it is often desirable to recover them from natural or industrial sources where such gases are in admixture with other gases. A variety of separation technologies exist for separation of rare and/or costly gases from gas mixtures, including adsorption, such as pressure swing adsorption (PSA), and cryogenic distillation.
One separation technology, gas separation membranes, typically includes one or more compressors and one or more gas separation membranes arranged in parallel or series. A permeate is obtained on a side of the membrane opposite the side to which the feed gas is fed. The separation layer of the membrane preferentially permeates one gas or gases in comparison to another gas or gases so that the permeate becomes enriched in one or more components. The non-permeate is obtained from the same side of the membrane from which the feed gas is fed and consequently is deficient in the component or components of which the permeate is enriched. Selection of the particular material making up the separation layer is driven by which components in the feed gas are desired for enrichment in the permeate and which components in the feed gas are desired for enrichment in the non-permeate. While there are a wide variety of materials used in gas separation membranes, one type of commonly used material is glassy polymers.
One gas of particular interest for recovery is Helium which is only available at significantly high volumes from natural gas. Helium is typically present in natural gas at below 0.5 mol % levels and is mostly extracted as crude Helium across liquid natural gas (LNG) trains. This crude Helium, containing about 20-30 mol % Helium, is then enriched either by cryogenic distillation or via a PSA to make 99.9999 mol % Helium.
Small gas molecules such as Helium are well known to be more permeable through glassy polymer membranes than methane or N2. Hence, membranes can be considered for Helium recovery from natural gas. However, Helium is typically found in very low concentrations and it is difficult for a single stage membrane to achieve commercially viable levels of recovery and/or selectivity. High Helium purity is desired because it reduces the cost of further processing and it limits the loss of the slower permeating natural gas
In general, recovery of dilute components by membranes requires multiple stages in order to achieve high purity. Other mass transfer operations, such as distillation can produce high purities by means of multiple stages. Unfortunately, membrane processes are expensive to stage since each additional stage often involves permeate recompression with the attendant operating and capital costs of the compressor. In other words, the permeate from the first stage typically must be compressed to a satisfactory pressure for separation in the second stage and the permeate from the second stage similarly may need to be compressed before it is fed to the third stage. Each additional compressor increases the capital, and especially operating, expense of such a multi-stage scheme.
Methods of optimally staging membrane processes have been extensively studied in the academic literature in an effort to reach a desired recovery and/or purity. Examples of this work include Agarwal, et al., (“Gas separation membrane cascades II. Two-compressor cascades”, Journal of Membrane Science 112 (1996) 129-146) and Hao 2008 (“Upgrading low-quality natural gas with H2S— and CO2-selective polymer membranes Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams”, Journal of Membrane Science 320 (2008) 108-122).
Staged membrane operations are also practiced commercially.
A prior art 2-stage configuration for He recovery is described in RU114423U where high pressure natural gas with a relatively small He content is fed to the feed side of the 1st stage membrane. The 1st stage membrane permeate (enriched in He) is re-compressed and fed to the feed side of the 2nd stage membrane. The permeate from the 2nd stage is further enriched in He and constitutes the product which may be re-compressed for further purification or usage. While this is a relatively simple configuration to operate, this configuration is limited in the final product He purity.
Higher He product purity can be achieved through a cascade of membrane stages. For example, by adding a 3rd stage which is fed by re-compressing the 2nd stage permeate, the product He purity can be further enhanced. In practice, such schemes are rarely used because of their added complexity and additional compressor cost. A pseudo-3-stage operation that does not require an additional compressor stage is taught in U.S. Pat. No. 7,604,681. However, the lower pressure ratio across the 2nd and 3rd membrane stages causes lower separation factors across these stages. With high selectivity membranes, such a scheme becomes pressure ratio limited and product He purity is reduced.
Permeate refluxing is described in some versions of membrane column work by Tsuru, et al. (“Permeators and continuous membrane columns with retentate recycle”, Journal of Membrane Science 98 (1995) 57-67). In this context, permeate refluxing is practiced on a single membrane stage with refluxing of a fraction of the permeate, then re-compressing that fraction and recycling it to either the feed gas or as a sweep gas. This permeate refluxing scheme is not appropriate for handling a high volume gas as the membrane area required for combined high purity and high recovery is very high.
A configuration incorporating a permeate recycle suitable for fast gas purification is described in the 2-stage process described by WO 12050816 A2. In this scheme, permeate from a first membrane stage (or from a section of a first membrane stage) is re-compressed and processed by a second stage consisting of 2 membrane banks in series. The second stage permeate is achieved at higher fast gas purity. In this scheme, the series stages serve as an overall second stage permeate reflux with permeate from the first in the series constituting the fast gas enriched product while permeate from second in the series is recycled to increase the fast gas concentration entering the second stage. The higher purity permeate from first in the two stages in series is the fast gas enriched product. The lower purity permeate from the second of the two stages in series is recycled to the suction of the compressor feeding the second membrane stage. The second stage non-permeate is recycled to the first stage membrane feed. Higher fast gas product purity from the first in the series can be achieved by reducing the membrane area in the first of the series relative to second of the series. However, in practice, the membrane area in the first of the series cannot be reduced markedly without incurring a high feed to non-permeate pressure drop. Such a high pressure drop is a parasitic energy loss, and with conventional membrane design, can pose a threat to the mechanical integrity of the membrane. The relative permeate rates from first in the series versus the second in the series are also difficult to adjust without extensive and complex plumbing. Thus, the relative rates would in practice be fixed and cannot be easily manipulated in order to adjust for varying feed concentrations, pressures or membrane performance changes.
It is therefore an object of the invention to provide a method and system for membrane-based gas separation to obtain a satisfactorily high recovery of a first gas at a satisfactorily high purity from a source gas that includes a minor amount of the first gas and a majority of a second without sacrificing too much of the second gas. It is also an object of the invention to provide a method and system for membrane-based gas separation to obtain a satisfactorily high recovery of the first gas at a satisfactorily high purity from the source gas without requiring an undesirably high gas separation membrane surface area. It is also an object of the invention to provide a method and system for membrane-based gas separation of first and second gases without a parasitic energy loss. It is also an object of the invention to provide a method and system for membrane-based gas separation of first and second gases that can be easily adjusted in view of varying feed concentrations, pressures, or membrane performance changes.